Here's the thing I have to remind myself of constantly: I'm in this for the long haul. This is all about feeling better, doing more, and living healthier for the long run (despite seasonal setbacks). It is easy to throw in the towel after such a setback. You've lost a lot of what you gained in terms of muscle mass and stamina. You've gained what you didn't want, weight. But remember, this is a lot more about remaining flexible, being able to do what you want to do for much longer (within reason based on age), and being there for family and friends for the long haul.
When you go back, you will once again discover who good it feels to exercise ... after the initial few minutes in which your body whines about how horrible you are being to it and wouldn't you rather go out somewhere for ice cream? Don't listen. Keep working. After a few minutes, the body realizes this really feels good and will get with the program.
However, there is one stipulation to all this. You can't let your ego rule the day. You can't try to return to the time and the weight you were running and lifting prior to the setback. You've got to go back to the beginning as they say in the Princess Bride and start with lower times and weights, unless you want to hurt yourself and spend even more time away from the gym. Your choice. I chose last night to shelve the ego and I feel much better today than I have for a month.
Good luck fellow fitness warriors out there. Take it slow, stick with it for the long run, leave the ego at the door, and remind yourself that unless you are under 25, you will never look like the muscle men and women on the magazine covers or in the movies. That's really not the point.